A cardiotonic directly acts on a heart to enhance the contraction thereof and various pharmaceutical drugs have been used in the treatment of cardiac insufficiency.
However, many of these cardiotonics have defects in that their safety margin is extremely narrow, they may cause arrhythmia, their cardiac stimulating activity is transient and they are not suitable for oral administration.
The present inventors have sought for compounds which are of high activity and capable of showing the sufficiently sustained activity as the cardiotonic and have discovered the present invention.